1. Field of the Invention
The present invention relates to an optical system for a retinal camera, and more particularly to an optical system for a retinal camera having a wide viewing angle and which does not generate flares or ghosts in pictures taken by the camera.
2. Description of Prior Art
In a retinal camera, it is desirable that the objective lens has a wide view field so that it can take a picture of a wide range of the eye fundus. Conventional optical systems for retinal cameras are generally designed so that the eye fundus is illuminated through the objective lens. With such an arrangement, however, the illuminating beam is reflected by each surface of the objective lens, generating flares or ghosts, so that the quality of the picture is lowered.
For such reason, there are provided black points or images of black points covering the area of passage of the illuminating beam at the predetermined surfaces of the objective lens, and said images of the black points are produced by black points provided conjugate with the predetermined surfaces of the objective lens with respect to relay lenses, so that harmful beams are prevented from being mixed in with the photography beam.
In conventional retinal cameras, among objective lenses there are known those comprised of a small number of lenses, for example, positive meniscus lens, a biconvex lens, and a combination of meniscus lens and a biconvex lens. However, the positive meniscus lens cannot be designed with a viewing angle greater than 30.degree.. The biconvex lens can have a viewing angle of about 50.degree., but it is difficult to correct various aberations. The combination of a meniscus lens and a biconvex lens cannot be designed so as to have a long working distance, meaning the spacing between the patient's eye and the objective lens.
To overcome the disadvantages of these lenses, the number of the lenses must be increased. If the number of the lenses is increased, the number of black points for preventing the harmful beam from mixing with the photographing beam must be increased, so that the illuminating beam or the photographing beam is decreased.